1. Field of the Invention
The present invention relates to a fuel cell, and more specifically to a fuel cell incorporating a metalized gas diffusion layer which is disposed in ohmic electrical contact with an adjoining current collector, and wherein a substantially force independent contact resistance is established between the current collector and the gas diffusion layer during fuel cell operation.
2. Background of the Invention
A fuel cell is a device which can readily convert chemical energy to electrical energy by the reaction of a fuel gas with a suitable oxidant supply. In a proton exchange membrane fuel cell, for example, the fuel gas is typically hydrogen, and the oxidant supply comprises oxygen (or more typically ambient air). In fuel cells of this type, a membrane electrode diffusion layer assembly is provided and which includes a solid polymer electrolyte with opposite anode and cathode sides. Appropriate electrodes are provided on the opposite anode and cathode sides. During operation, a fuel gas reacts with a catalyst present in the electrode on the anode side to produce hydrogen ions which migrate through the solid polymer electrolyte to the opposite cathode side. Meanwhile, an oxidant supply introduced to the cathode side is present to react with the hydrogen ions in the presence of catalyst which is incorporated into the electrode on that side, to produce water and a resulting electrical output.
Many fuel cell designs have been provided through the years, and much research and development activity has been conducted to develop a fuel cell which meets the perceived performance and cost per watt requirements of various users. Despite decades of research, fuel cells have not been widely embraced except for narrow commercial applications. While many designs have emerged, and which have operated with various degrees of success, shortcomings in some peculiar aspect of their individual designs have resulted in difficulties which have detracted from their widespread commercial acceptance and perceived usefulness. For example, one of the perceived challenges for fuel cell designers is the reduction of contact resistance between the current collector and an adjacent gas diffusion layer and which is borne by the membrane electrode diffusion layer assembly. This contact resistance is generally speaking inversely related to the current output of the fuel cell. Consequently, lowering the contact resistance increases the overall current output of the fuel cell.
Heretofore, the conventional prior art means of minimizing this contact resistance has been to apply relatively large amounts of direct pressure or force to the current collector which lies in immediate, ohmic electrical contact with the gas diffusion layer. The designers and manufacturers of both fuel cell stacks, and fuel cell modular power systems include in their respective fuel cell designs, various force application assemblies to transmit force directly to the current collectors to minimize contact resistance. These force application assemblies have included various mechanical schemes and other arrangements to apply force or pressure substantially evenly across the surface area of the membrane electrode diffusion layer assembly enclosed within the fuel cell. While these several schemes have operated with some degree of success, there have been inherent shortcomings which have continued to detract from their usefulness.
As a first matter, these above mentioned mechanical arrangements for applying force provide an additional level of complexity, and thus cost to the design and manufacture of such fuel cell devices. Secondly, such force application assemblies increase the perceived difficulty in repairing, maintaining and modifying such devices. For example, if an individual membrane electrode diffusion layer assembly in a fuel cell stack begins to decline in performance, or fails, the fuel cell stack must often be taken out of service, and off-line. Subsequently, the fuel cell stack must be completely disassembled in order to replace the failed component. This disassembly includes the removal of several long tie or compression bolts; disassembly of the fuel cell stack; and then the reassembly of the fuel cell stack followed by the precise application of torque to each of the same tie bolts to reestablish the appropriate amount of force applied by the stack to the plurality of membrane electrode diffusion assemblies. Further, in fuel cell stack arrangements, the pressure applied by these same tie bolts is also necessary in order to establish an appropriate amount of sealing force throughout the stack and thus prevent the escape of hydrogen; coolant; oxygen; or the by-product, water.
In a modular fuel cell power system such as what has been shown and described in U.S. Pat. No. 6,030,718, and in our pending application Ser. Nos. 09/873,139; 09/577,407 and that application filed Nov. 13, 2001, assorted fuel cell modules have been described and claimed and which utilize individual membrane electrode diffusion layer assemblies which are oriented in predetermined arrangements such that the respective modules can be readily removed from a subrack, for example, while the remaining modules continue to operate. In each of these fuel cell designs, the load which is electrically coupled to the power system continues to be substantially electrically served, and the failed fuel cell module can be readily replaced and/or serviced without the inconvenience of taking the entire fuel cell power system out of service, and off-line, as is the current prior art practice.
In addition to the shortcomings noted above, if force or pressure is not evenly applied across the face of a membrane electrode diffusion layer assembly, the fuel cell will not reach it""s full potential for producing electrical power. As will be recognized, all of the foregoing have detracted from the widespread acceptance of fuel cells for use in various commercial applications.
These and other perceived shortcomings are addressed by means of the present invention.
Therefore, one aspect of the present invention is to provide a fuel cell which includes an ion exchange membrane; an electrode positioned in ion exchanging relation relative to the ion exchange membrane; a gas diffusion layer borne by the electrode and having an outwardly facing surface; a porous metal coating comprising one or more elements selected from the periodic table of elements and which has an atomic number of less than 75, and which is positioned at least in partial covering relation relative to the outwardly facing surface of the gas diffusion layer; and a current collector forcibly disposed in ohmic electrical contact with the porous metal coating.
Another aspect of the present invention is to provide a fuel cell which includes an ion exchange membrane; an electrode borne by the ion exchange membrane; a gas diffusion layer borne by the electrode and having an outwardly facing surface area; and a current collector forcibly juxtaposed relative to the gas diffusion layer, and wherein, during fuel cell operation, a contact resistance is created between the gas diffusion layer and the current collector, and which is expressed in milliohms per square centimeter of surface area, and wherein pressure applied by the current collector is expressed in pounds per square inch of surface area, and wherein the contact resistance, and the pressure applied by the current collector can be individually mathematically expressed as a logarithm, and wherein the relationship of the logarithm of the contact resistance and the logarithm of the pressure applied by the current collector is defined by a line having a slope of about 0 to about a negative 0.8.
Still another aspect of the present invention relates to a fuel cell which includes an ion exchange membrane; an electrode disposed in at least partial covering relation relative to the ion exchange membrane; a gas diffusion layer borne by the electrode, and which has an outwardly facing surface area, and which further has a porous metal coating applied thereto; and a current collector juxtaposed relative to the metal coating, and wherein a pressure of less than about 300 pounds per square inch is applied by the current collector to the juxtaposed gas diffusion layer and which creates a contact resistance, during fuel cell operation, of less than about 20 milliohms per square centimeter between the porous metal coating and the current collector, and wherein the fuel cell, during operation, generates a substantially constant electrical current when exposed to a pressure of less than about 300 pounds per square inch by the current collector at operating temperatures of less than about 300xc2x0 C.
These and other aspects of the invention will be discussed in further detail hereinafter.